In preparation of wall panels, ceiling boards, floor tiles, or planks for large furniture, widely used conventional methods are joining small hard boards to form a larger plank as desired using adhesive and/or nails which is complicated and requires tool for lamination, or tongue insertion carried out by forming a tongue and a tongue groove and joining using adhesive for horizontal fixing, scarfing, overlapping, pinning, using additional equipment for fixing, and other approaches for achieving said objective. Examples of limitation as shown in prior art patents are as follows.
Examples of hard boards with engaging parts include the inventions as described in the U.S. Pat. No. 6,766,622 B1 and U.S. Pat. No. 7,698,868 B2 disclosing hard boards having engaging parts for fixing without using additional equipment, International Patent Application Publication No. WO1994026999 A1 and International Patent Application Publication No. WO1996027721 A1 disclosing hard boards having engaging parts for fixing using additional equipment, European Patent No. EP 1084317 B2 disclosing hard boards having engaging parts for fixing with slight modification or addition in the engaging parts e.g. contact surface angle and thickness, U.S. Pat. No. 6,209,278 B1 and U.S. Pat. No. 6,216,409 B1 disclosing hard boards having engaging parts for fixing wherein the engaging parts were configured into two zig-zag grooves being in contact and complementary to each other, and the like. However, the hard boards according to said inventions often have limitations in use or manufacture as follows.                The hard board must have a firm shape, straight and planar surfaces, a uniform thickness across the board, and particularly a flexibility property as joining the hard boards requires spreading and closing in of the male connector and the female connector for engagement. Therefore, the material used preferably have a high flexibility, a thickness of about 3-12 mm, and is not susceptible to crack or breakage. This type of hard board is therefore preferably produced from a flexible hard board formed by industrial processed into flat board e.g. plastic boards, wood-plastic composite boards, plywood boards, HDF (High Density Fiber), MDF (Medium Density Fiber) hard boards, or PBs (Particle Board), and the like.        The engagement of the male connector and the female connector requires specifying a reference point and rotating the male connector about said reference point in circle with said reference point as the center thereof until an obstacle is reached. Then the female connector is spread and closing in for engagement. In some cases, additional equipment produced for other industries may be used to facilitate the engagement. With the circumferential rotation of the male connector, the joining of the male connector and the female connector is tight but a spread-and-close engagement is caused. One disadvantage of this principle is the loss of flexibility property occurring when disassemblies and reassemblies are carried out which means that further engagements will be loose.        For flexibility in the engagement and for a stable fixation of the hard boards, the hard board's thickness must be not too high or too low and normally in a range of 3 millimeters to 12 millimeters and        The machine used in manufacture requires a motor shaft for fitting a cutting tool for an angle-adjustable forming which is costly and requires complicated work setting which may result in inaccurate and defected products.        
However, none of the patents and publications disclosed an invention which provides a hard board which has no flexibility, no thickness limitation, no limitation in the types of material e.g. iron, and particularly lumber; applies the principle of moving along the circumferences of three ellipses of the male connector without requiring flexibility property for spreading and closing in the engaging parts; can be assembled easily without using adhesive and nails and any additional equipment; and can be disassembled and reassembled as many times as desired without losing horizontal and vertical fixing property.